1. Field of the Invention
This invention relates to a multi-value recorded data detecting method suitably applied to reproduction of multi-value data recorded on a record medium such as an optical disk.
2. Description of the Related Art
A record medium such as, for example, an optical disk or an opto-magnetic disk has spiral or concentric tracks on which two-value or three- or more-value digital data are recorded in the form of concave or convex pits formed by by embossing or a like technique or in the form of different magnetization directions.
When the recorded data are to be reproduced, a laser beam is irradiated upon a track and a difference in intensity of the reflected light or a difference in polarization direction of the reflected light caused by a magnetic Kerr effect is detected to obtain a reproduction RF signal. Then, two-value or three- or more-value data are detected from the reproduction RF signal thus obtained.
FIG. 7 shows in block circuit diagram a reproducing circuit for detecting three-value data from an optical disk on which data are recorded in three values, and FIG. 8 shows waveforms at different portions of the circuit of FIG. 7. Referring first to FIG. 7, data are recorded in a predetermined track format on an optical disk 1, and the optical disk 1 is driven to rotate at a constant linear velocity (CLV) or a constant angular velocity (CAV) by a spindle motor 2. An optical head 3 for irradiating a laser beam upon recording or reproduction is disposed below the optical disk 1.
The optical head 3 includes an optical system including a laser light source, a collimator lens, a beam splitter, a two axial device for controlling an objective lens and so forth, and further includes a polarized beam splitter and a detector for detecting reflected light from the optical disk 1. Detection information from the detector which detects such reflected light is supplied as an electric signal to a calculating and amplifying circuit 4. The calculating and amplifying circuit 4 calculates the received detection information and generates a tracking servo signal, a focusing servo signal and a reproduction data signal. The tracking servo signal and the focusing servo signal are supplied to a servo controlling circuit 5. The servo controlling circuit 5 performs tracking and focusing servo operations of the optical head 3 in accordance with the thus received tracking servo signal and focusing servo signal.
Meanwhile, the reproduction data signal is amplified by an RF amplifier 6 and then inputted to an analog to digital (A/D) converter 7. A sampling clock signal CK for the analog to digital converter 7 is generated by a PLL (phase-locked loop) circuit 8, and predetermined threshold values L.sub.S1 and L.sub.S2 are supplied to a multi-value converting circuit 9. It is to be noted that reference characters A to E correspond to curves A to E of the waveform diagram of FIG. 8.
Now, if, for example, such three-value data including "0", "1" and "-1" as seen from the representation of FIG. 8A are recorded on the optical disk 1, a reproduction RF signal amplified by the RF amplifier 6 presents such a waveform as shown by the curve of FIG. 8B and is inputted to the analog to digital converter 7. At the analog to digital converter 7, the reproduction RF signal is sampled in accordance with such a sampling clock signal CK supplied thereto from the PLL circuit 8 as seen from the curve of FIG. 8C so that it is converted into digital codes. Here, if it is assumed that the output of the analog to digital converter 7 is 8-bit data and consequently the dynamic range of the analog to digital converter 7 is divided into 256 steps, then the signal level data of the reproduction RF signal are converted into such digital codes, for example, as seen from the curve of FIG. 8D, and the digital codes thus obtained are outputted from the analog to digital converter 7.
The output of the analog to digital converter 7 is inputted to the multi-value converting circuit 9, to which the predetermined threshold values L.sub.S1 and L.sub.S2 are supplied. Thus, the signal level data digitally coded into 256 steps as described above are individually compared with the threshold levels L.sub.S1 and L.sub.S2 at the multi-level converting circuit 9 so that they are classified into the three values of "0", "1" and "-1" as seen from the curve of FIG. 8E.
After three-value recorded data are detected in this manner, predetermined demodulating processing is performed for the thus detected data by performing error correcting processing and so forth in accordance with an ECC (Error Correction Code) included in the data.
By the way, a reproduction signal from such an optical disk as described above often suffers from a variation in level or a variation in amplitude arising from various factors including a difference in reflection factor between different optical disks or a difference in reproduction frequency characteristic between reproduction signals on the inner circumference side and the outer circumference side of a same optical disk.
If the threshold values upon multi-value conversion after analog to digital conversion are fixed while a level variation or an amplitude variation takes place with a reproduction signal in this manner, then the reproduction signal may be detected as an wrong value. Particularly where the data are three- or four- or more-value data, the number of required threshold values increases correspondingly, and the probability of wrong correction of multi-value data increases as much.
Since such detection error upon multi-value conversion cannot be corrected by error correcting processing based on an ECC at a following stage, accurate detection of multi-value data is required.